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March 27, 2012
Broken Breakers, Part 5
Electrical Troubleshooting Quiz
Replacing Cables, Part 8
NEC in the Facility
Safety
Electric Motor Maintenance and Troubleshooting

Maintenance


Broken Breakers, Part 5

When the breaker maintenance procedure says "inspect contacts," do more than just verify their presence. Before examining the breaker contacts, inspect the breaker cubicle. If the breaker operated under load, then there's a good chance you'll see "welding slag" on the bottom or walls of the enclosure. The metallic particles may not be readily visible, so shine a light at various angles to help spot these.

Because this metal coating is not part of the original design, it needs to be removed after sufficient lockout/tagout. Use a soft-bristled brush and vacuum; make slow strokes with the brush so you aren’t creating any clouds of metal particles. If the material still won't come off, make a note to clean the enclosure during the next outage rather than use volatile solvents when the surrounding switchgear is energized.

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Repair


Electrical Troubleshooting Quiz

In a production area less than a year old, an 800A feeder supply breaker tripped. Before attempting a reset, you performed insulation resistance tests on the conductors. Resistance was low enough to indicate the insulation had melted.

Obviously, the breaker mechanism wasn't frozen or the breaker wouldn't have opened at all. Your electrical testing firm ran a complete diagnostic on the breaker, and it performed flawlessly. A review of the power monitoring logs showed no excessive current until just before the breaker tripped. This indicates that the melting caused the excess current, not the other way around.

What should you examine next?

Visit EC&M's website to see the answer.

Replacing Cables, Part 8

To keep cables attached during a cable pull, you join the cable to the pulling lead using a knot. You wrap tape around the knot to "smooth out" the joint so it slides more easily through the raceway.

The tape isn't intended to hold the joint together. Wrapping excess tape around the joint to "make up for" a poor knot is a mistake. As the diameter of the splice grows, so does the force required to pull it through the raceway. This joint isn't nearly as flexible as the joined cable and pulling lead, so it's the binding point of the entire pull.

You want to keep this joint as small as possible but strong enough that it won't pull apart. How can you achieve this?

To read more on this story, visit EC&M's website.

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Operation


NEC in the Facility

Section 110.24 is new to the NEC as of the 2011 revision. Installers now must mark service equipment (other than residential) with the available fault current. The marking must include the date of the fault current calculation, and it must be durable.

In other requirements for marking, the NEC explicitly says legible. That isn't stated here. However, since it makes no sense to provide illegible markings, legibility is inherently required.

A good way to provide a durable, legible marking is to create a laminate material sign that you affix with screws. Such signs are often used to identify equipment, post warnings, and provide other information. So the means of making this sign are usually already onsite or readily available.

To read more on this story, visit EC&M's website.

Safety

Corrosive chemicals can cause severe damage to your skin. If you inhale them, they can destroy your lungs. How do you know if a chemical is a corrosive? First of all, the shipping container is always marked "corrosive" and bears the DOT sign for corrosives. The individual product container will also have this sign, so look for it.

A corrosive chemically reacts with other substances (the result is a salt) via ionic exchange. This reaction can be quite violent. The higher or lower the pH (a measure of free ions of hydrogen) is, the more dangerous the corrosive. Look for the pH on the MSDS.

The pH of pure water is 7.0, which means it’s neutral. The further the pH is from 7, the more corrosive the chemical is. Acids have a low pH; hydrochloric acid has a pH of 0. Bases have a high pH (the maximum is 14). You can feel an acid burn your skin, but with a base you can't feel the burn.

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Books


Electric Motor Maintenance and Troubleshooting

Keep electric motors running at their peak performance level. Electric Motor Maintenance and Troubleshooting, Second Edition, explains in detail how all types of AC and DC motors work. Essential for anyone who needs to buy, install, troubleshoot, maintain, or repair small to industrial-size electric motors, this practical guide contains new information on three-phase motors along with coverage of the latest test instruments.

Drawing on his more than 40 years of experience working with electric motors, expert author Augie Hand provides a wealth of tested procedures to pinpoint and correct any kind of issue. He'll help you decide whether to replace a motor, take it offline for repair, or repair it in place--decisions that can reduce down time. End-of-chapter questions reinforce the material covered in the book. Quickly and accurately diagnose electric motor problems and find effective solutions with help from this fully updated classic.

Currently available for purchase via EC&M Books.

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